1. Field of the Invention
The present invention relates to a speaker driving device and audio output system that drive a speaker mounted in a portable device such as a cellular phone, for example, by means of an output signal obtained by amplifying an input signal.
2. Description of the Related Art
Conventionally, in the case of a device in which a speaker is mounted, i.e. a portable device such as a digital video camera (DVC), digital still camera (DSC), cellular phone, or notebook personal computer, for example, when a speaker driving device that drives the speaker takes the form of an IC and constitutes an audio output system together with the speaker, there is a multiplicity of cases where a piezoelectric speaker is used with the objective of a small footprint as the mounted speaker.
A conventional speaker driving device for driving such a piezoelectric speaker and an audio output system will be described hereinbelow taking the example of the speaker driving device and audio output system that appear in Japanese Patent Application Laid Open No. S63-217806.
FIG. 5 is a block diagram showing the constitution of an audio output system that drives a piezoelectric speaker by means of a conventional speaker driving device. In a speaker driving device 2 shown in FIG. 5, an amplifier circuit 8 that is capable of driving a piezoelectric speaker constituting a speaker 1 which is connected to output terminals 6 and 7 by means of a high dynamic range in order to obtain an adequate acoustic pressure level by amplifying an input signal from input terminal 5, and a power supply switching circuit 9 for generating a high voltage power supply that is required for the output stage of the amplifier circuit 8 by increasing the voltage from an external low voltage power supply are incorporated on the same chip. As a result, miniaturization of the device and ease of mounting were surveyed.
However, in a conventional speaker driving device and audio output system as described above, because the output power from the amplifier circuit 8 is inversely proportional to the load impedance of speaker 1, the piezoelectric speaker can be suitably driven without design problems. However, assuming a case where a dynamic speaker that is generally widely used to stabilize costs, for example, is used with this circuit constitution as is, a speaker with a lower load impedance than that of a piezoelectric speaker is driven, in the case of such speaker driving, the output power from the amplifier circuit 8 is large in comparison with a case where a piezoelectric speaker is driven, which results in the problem that the acoustic pressure from the speaker is excessive.
That is, the conventional speaker driving device and audio output system shown in FIG. 5 are unable to adapt without operational distinction to both driving types of the piezoelectric speaker and dynamic speaker with a different load impedance as driven speaker 1 by means of the same constitution and are unable to output a signal of the same power for the same input signal in the respective speaker driving.
In view of this problem, as a constitution for outputting a signal of the same power for the same input signal by considering the fact that the load impedance is different for the dynamic speaker and piezoelectric speaker, for example, in the driven speaker, the constitution of an audio output system comprising a speaker driving device shown in FIG. 6A or FIG. 6B may be considered.
Audio output systems comprising a speaker driving device shown in FIG. 6A and FIG. 6B will be described hereinbelow.
FIG. 6A and FIG. 6B each is a block diagram showing another constitutional example of an audio output system comprising a conventional speaker driving device. In the audio output systems shown in FIG. 6A and FIG. 6B, 600 is a speaker driving device having a plurality of amplifier circuits of different amplification gains corresponding with the types of driving load; 601 is a power supply terminal for applying power (VCC) that supplies the power consumed during signal amplification to the speaker driving device 600; 602 is a driving load switching terminal for applying a switching signal for selectively switching the type of amplifier circuit that acts in the speaker driving device 600 in accordance with the type of driving load; 603 is an output terminal that outputs supplied power that is supplied to the driving load; 604 is a dynamic speaker, which is one of the driving loads; and 605 is a piezoelectric speaker, which is another one of the driving loads.
The operation of the audio output system comprising the speaker driving device constituted as described above will be described next.
As mentioned earlier, the speaker driving device 600 shown in FIG. 6A and FIG. 6B comprises built-in amplifier circuits (not shown) of two types with different amplification gains so that the output power to each of the loads 604 and 605 are equal for the same input signal level corresponding to the type (local impedance) of the connected loads 604 and 605 and the amplifier circuits of two types are switched according to the types of the loads 604 and 605 by means of the switching control signal high (VCC) or low (0V) applied from the outside to the driving load switching terminal 602.
For example, as shown in FIG. 6A, the constitution is such that, when the dynamic speaker 604 having a low load impedance is connected as the load to the output terminal 603 of the speaker driving device 600, by applying a power supply of VCC=2.8V as the power supply to the speaker driving device 600 to the power supply terminal 601 and making the potential of the switching control signal applied from the outside to the driving load switching terminal 602 a low potential (0V), selective switching is controlled such that the amplifier circuit with an amplification gain for driving the dynamic speaker 604 operates as a connected load among the two types of amplifier circuit in the speaker driving device 600.
On the other hand, as shown in FIG. 6B, the constitution is such that, when the piezoelectric speaker 605, which has a higher load impedance than the dynamic speaker 604, is connected as the load to the output terminal 603 of the speaker driving device 600, by applying a power supply of VCC=5V as the power supply to the speaker driving device 600 to the power supply terminal 601 and making the potential of the switching control signal applied from the outside to the driving load switching terminal 602 a high potential (VCC), selective switching is controlled such that the amplifier circuit with an amplification gain for driving the piezoelectric speaker 605 operates as a connected load among the two types of amplifier circuit in the speaker driving device 600.
By controlling the selective switching in this manner, the output power to each load is equal for the same input signal level, irrespective of the type of connected load.
As a result of such a constitution, the above problem is solved. However, in the audio output system comprising the speaker driving device shown in FIG. 6, the constitution is such that selective switching is controlled to select an amplifier circuit with an amplification gain corresponding with the connected load among the two types of amplifier circuit in the speaker driving device 600 by applying a control signal from the outside in accordance with the type of the load connected to the output terminal 603 of the speaker driving device 600 as mentioned earlier and, therefore, the audio output system must comprise a special control terminal for selectively switching from the outside the two types of amplifier circuits in the speaker driving device 600 corresponding to a dynamic speaker connection and a piezoelectric speaker connection as the speaker load. When the speaker driving device is made into an IC chip as a semiconductor integrated circuit, the total number of terminals must be a number of terminals that takes into consideration control-terminal expansion such as a driving load switching terminal, which creates a new problem that the size of the IC chip must be increased.